A couple months ago I learned about the elevated bicycling wood road from Pasadena to Los Angeles, http://justacarguy.blogspot.com/2013/07/forgotten-history-of-what-might-have.html and today Randy sent me an email about these large concrete markers that the airmail pilots used to stay on course from New York to San Fransisco, before electronic stuff like radar and radio direction finding, when all a pilot had was a compass and a map... and those weren't so great to use while airborne.

So these concrete arrows were made, painted yellow for daylight visibility, and lit with enormous light towers at night

the two maps below indicate the airfields used for refueling and food stops, not the arrows.